The present invention relates to RF receivers having a fixed frequency IF section, and more particularly to correction of the frequency response of an RF receiver having a frequency translation device.
Traditionally the normalization process or alignment process, i.e., self-calibration, for an RF receiver includes measuring a channel frequency response both in magnitude and phase by using a stimulus signal, usually generated internally, applied to an input of the receiver. It is assumed that the frequency response of the RF receiver front end having a frequency translation device is relatively flat over the IF bandwidth, i.e., the IF bandwidth response does not change with change in center frequency. Therefore the normalization process is only performed using the stimulus signal at a single reference frequency. When the device operates at other RF frequencies, the differences in frequency responses over the IF bandwidth at the other channel or center frequencies is ignored.
Frequency translation devices, such as mixers that combine an input signal with a frequency from a local oscillator to translate the input signal to a different frequency, are important components in radio frequency (RF) systems. The conversion by the mixers of RF power at one frequency into power at another frequency makes signal processing at a receiver easier and more efficient. For modern RF systems minimizing linear distortion along a signal path, including contributions from the frequency translation devices, is critical to meet signal quality requirements, such as Error Vector Magnitude (EVM) for communication systems.
In the RF receiver the mixer is just one building block in any signal path. Other cooperating circuits need to be considered since the mixer interacts with both the preceding and subsequent circuits in the signal path, as well as local oscillator (LO) driving circuits. All mixers are not fully isolated, leading to leakage or feed-through between mixer ports. Also mixers are non-linear devices which inevitably result in inter-modulation distortion, leading also to a variable frequency response across the IF bandwidth. In traditional narrow bandwidth systems, such as 1-10 MHz intermediate frequency (IF) bandwidth systems, the mixers may be approximated as having ideal frequency responses, i.e., flat gain and linear phase over the mixer bandwidth, without causing too much degradation in system performance. RF receivers, such as those in traditional spectrum analyzers, make this assumption and only perform channel alignment at the center frequency.
However for wide bandwidth systems, such as RF receivers having signal paths with an IF bandwidth of 100 MHz and greater, the interaction between the mixer and cooperating circuits gets more complicated. Considerable errors occur if the mixer is only calibrated as having an ideal frequency response. This results in errors being generated in RF receivers. In test equipment having RF receivers such errors may result in the inability to reliably measure whether or not a device being tested meets required specifications.
What is desired is a method of correcting the frequency response of an RF receiver having a frequency translation device in a signal path over a wide IF bandwidth.